US10974984B2 - Manufacturing process for striae-free multicomponent chalcogenide glasses via convection mixing - Google Patents

Manufacturing process for striae-free multicomponent chalcogenide glasses via convection mixing Download PDF

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US10974984B2
US10974984B2 US16/226,871 US201816226871A US10974984B2 US 10974984 B2 US10974984 B2 US 10974984B2 US 201816226871 A US201816226871 A US 201816226871A US 10974984 B2 US10974984 B2 US 10974984B2
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glass
zone
furnace
ampoule
temperature gradient
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US20190194052A1 (en
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Vinh Q. Nguyen
Jasbinder S. Sanghera
Daniel J. Gibson
Mikhail Kotov
Gryphon A. Drake
Shyam S. Bayya
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US Department of Navy
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    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
    • C03B5/16—Special features of the melting process; Auxiliary means specially adapted for glass-melting furnaces
    • C03B5/235—Heating the glass
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B5/00—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture
    • C03B5/06—Melting in furnaces; Furnaces so far as specially adapted for glass manufacture in pot furnaces
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B32—LAYERED PRODUCTS
    • B32B—LAYERED PRODUCTS, i.e. PRODUCTS BUILT-UP OF STRATA OF FLAT OR NON-FLAT, e.g. CELLULAR OR HONEYCOMB, FORM
    • B32B17/00—Layered products essentially comprising sheet glass, or glass, slag, or like fibres
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C1/00—Ingredients generally applicable to manufacture of glasses, glazes, or vitreous enamels
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03C—CHEMICAL COMPOSITION OF GLASSES, GLAZES OR VITREOUS ENAMELS; SURFACE TREATMENT OF GLASS; SURFACE TREATMENT OF FIBRES OR FILAMENTS MADE FROM GLASS, MINERALS OR SLAGS; JOINING GLASS TO GLASS OR OTHER MATERIALS
    • C03C3/00—Glass compositions
    • C03C3/32—Non-oxide glass compositions, e.g. binary or ternary halides, sulfides or nitrides of germanium, selenium or tellurium
    • C03C3/321—Chalcogenide glasses, e.g. containing S, Se, Te
    • C—CHEMISTRY; METALLURGY
    • C03—GLASS; MINERAL OR SLAG WOOL
    • C03B—MANUFACTURE, SHAPING, OR SUPPLEMENTARY PROCESSES
    • C03B2201/00—Type of glass produced
    • C03B2201/80—Non-oxide glasses or glass-type compositions
    • C03B2201/86—Chalcogenide glasses, i.e. S, Se or Te glasses

Definitions

  • the present invention relates to making striae-free multicomponent chalcogenide glasses with uniform refractive index using convection mixing.
  • Chalcogenide glasses comprise at least one chalcogen element (S, Se or Te) and other elements including, but not limited to, Ge, As, Ga, Sn, Sb and transmit infrared light (IR) from between about 1 ⁇ m, or lower, to about 12 ⁇ m or greater, depending on composition.
  • IR infrared light
  • chalcogenide glass fibers may be used for IR missile warning systems and laser threat warning systems to provide superior aircraft survivability, and high energy IR power delivery using for example, but not limited to, CO (5.4 ⁇ m) and CO 2 (10.6 ⁇ m) lasers (Sanghera et al., “IR fiber optics development at the naval Research Laboratory,” SPIE, 3950, 180-185 (2000) and Sanghera et al., “Applications of Chalcogenide Glass Optical Fibers at NRL,” J. Optoelectronics and Advanced Materials, 3, No. 3, 627-640 (2001)).
  • these fibers may be used for remote fiber optic chemical sensor systems for military facility clean-up and other industrial applications.
  • Chalcogenide glasses may also be used as bulk optical elements, including windows, lenses, prisms, beam splitters and the like, and must have high compositional uniformity and homogeneity in order to maintain accurate control of light rays passing through the glass and to achieve satisfactory optical results.
  • the properties of the chalcogenide-based glasses including optical, physical and thermal properties, such as refractive index, dispersion, thermo-optic coefficient, glass transition temperature, viscosity profile, hardness, fracture toughness, thermal expansion, density, nonlinear index, fluorescence, wavelengths of transmission and others, can be tailored through composition.
  • some chalcogenide glass compositions with technologically useful properties may be thermodynamically unstable whereby crystallites or other inhomogeneities, including phase-separated glassy regions or devitrified regions, form within the glass during synthesis, melting or processing.
  • thermodynamic instability limits the physical size of the glass that may be fabricated (such as Ge 30 As 22 Se 23 Te 25 ), and in some cases optical quality glass may not be made in any size due to crystal formation (such as Ge 13 As 32 Se 25 Te 30 ) (Kokorina, Glasses for Infrared Optics, CRC Press, Inc. (1996)). It is well-known in the art of glass making that thermodynamically unstable glasses can be synthesized by rapidly cooling the melt, but the glasses are not optical quality due to striations that form upon rapid cooling.
  • chalcogenide glasses inside of sealed containers, such as a quartz or silica glass ampoule, that has been evacuated or purged with an inert gas, such as nitrogen, argon, or similar.
  • an inert gas such as nitrogen, argon, or similar.
  • an inert crucible can be placed inside the silica ampoule to prevent contact of the melt with the ampoule if there is a detrimental interaction or chemical reaction between them.
  • inert materials include vitreous carbon, graphite, and other suitable materials.
  • the sealed vessel contains the gaseous chemical elements under positive pressure and prevents excessive evaporation.
  • a common practice in the synthesis and melting of chalcogenide glasses is to rock, invert, shake gently, or otherwise agitate the ampoule and the liquefied glass melt contained within in order to thoroughly mix or homogenize the glass melt.
  • chemicals in the vapor phase above the melt must condense as the equilibrium partial pressure decreases, and it is common that the resulting condensate deposits on the surface of the glass melt.
  • This condensate can be of varying chemical composition, dictated by the specific vapor pressure-temperature curves of the elements, resulting in a compositional inhomogeneity near the surface of the glass melt. It is common practice to remove and discard the top surface of the glass boule to remove condensation and the depletion layer.
  • the glass melt may be very fluid at the melt temperature, and either the condensate, the depletion layer, or both may percolate deeper within the glass boule as it solidifies carrying with it inhomogeneity in the form of striae or cord due to variations in chemical composition. This is especially true for glasses with multiple chemical components or with multiple stable phases.
  • An annealing step heating the glass for an extended period at a temperature near the glass transition temperature, is typically performed to alleviate stress in the glass, which can be another source of strain-induced optical inhomogeneity or stress birefringence. It is well understood however that the viscosity of the glass is sufficiently high during annealing that compositional inhomogeneity cannot be removed by annealing. Such glasses are difficult or impossible to fabricate with high homogeneity using the prior art methods.
  • the aforementioned problems are overcome in the present invention which provides a method for synthesizing high optical quality multicomponent chalcogenide glasses without refractive index perturbations due to striae, phase separation or crystal formation using a sealed ampoule with chemical components enclosed inside, a two-zone furnace, a convection heating/mixing step, and multiple fining steps.
  • the sealed ampoule is oriented vertically within the two-zone furnace and heated to melt the chemical components contained within, and a temperature gradient (in the vertical direction) is created between the top zone and the bottom zone such that the bottom zone has a higher temperature. This temperature gradient causes convection currents within the viscous liquid until it is sufficiently mixed due to the convective flow.
  • the temperature gradient is reversed such that the top zone now has a higher temperature and the convective flow ceases.
  • the furnace temperatures are then reduced over a period of time, with holds at multiple temperatures for fining (removal of bubbles) and cooling to form a solid glass.
  • the present invention provides a method for the synthesis of high-purity chalcogenide glasses with excellent clarity and no apparent optical inhomogeneity or striae without the need for mechanical agitation, stirring or rocking.
  • the glasses of this invention may be used for infrared optical elements, lenses, windows and infrared optical fibers.
  • the present invention enables synthesis of homogeneous, optical quality glasses for some glass compositions that are not possible using methods of the prior art.
  • the chalcogenide glasses and fibers described herein, and more specifically glasses and fibers containing primarily arsenic, sulfur, selenium, tellurium, and germanium with dopants including antimony, gallium aluminum, indium, bismuth, tin, iodine, bromine, chlorine, fluorine, lanthanum, and other elements up to about 10% atomic each, may be synthesized according to the method of the present invention in forms suitable for optical quality fibers and geometric optics including windows, lenses and other devices.
  • FIG. 1 shows vapor pressure (in Pascal, Pa.) curves of five elements commonly used in the production of chalcogenide glasses for infrared applications: sulfur, selenium, arsenic, tellurium, and germanium as a function of temperature in degrees centigrade (° C.).
  • FIG. 2 is a schematic overview of the glass melting and homogenization step of the process to synthesize infrared glasses by melt processing using the present invention, specifically without mechanical agitation.
  • a sealed ampoule containing a glass melt is inside a vertical furnace having two independently controllable heaters with the top zone set to a temperature about 350° C. lower than that of the bottom zone to provide a negative temperature gradient. Convection currents and evaporation-condensation provide a means of mixing the melt.
  • FIG. 3 is a schematic overview of the fining and cooling steps of the present invention.
  • a sealed ampoule containing a well-mixed, homogenized glass melt is inside a two-zone furnace oriented so as to have a common vertical central axis.
  • the temperature profile is measured using thermocouples along the length of the ampoule.
  • the upper heater was set to a temperature about 100° C. higher than that of the lower heater to ensure a positive temperature gradient.
  • FIG. 4 shows temperature profiles of the furnace top zone, furnace bottom zone, and the temperature gradient during the synthesis of As 39 Se 61 glass described in Example 1, Steps 1-7.
  • FIG. 5A shows a photograph of a sealed ampoule containing a solid As 39 Se 61 glass ingot with no soot or glass adhered to the ampoule above the ingot. The top and bottom of the ingot were removed at the dashed lines.
  • FIG. 5B shows the resulting cylinder from the removal of the ingot top and bottom. The end faces of the resulting cylinder were ground and polished flat and parallel.
  • FIG. 7 is a schematic representation of an As—Se glass cylinder showing the locations where the glass transition temperature was measured.
  • FIG. 8 shows temperature profiles of the furnace top zone, furnace bottom zone, and the temperature gradient during the synthesis of As 39 Se 61 glass described in Example 2, Steps 1-4.
  • FIG. 9 is a schematic overview of the glass melting and homogenization steps used in the prior art (Examples 3, 4, and 5).
  • a sealed ampoule containing a glass melt is inside a rocking furnace with a ⁇ 45° angle of inclination.
  • FIG. 10 is a schematic diagram of the glass fining and cooling steps of the prior art described in Example 3.
  • a sealed ampoule containing a glass melt is inside a rocking furnace in a vertical position. Thermal gradients within the furnace cause thermal convection currents in the glass melt, and evaporation and condensation that drip back into the melt cause striae.
  • FIG. 11 is a photograph of a sealed ampoule containing a solid ingot of As 39 S 61 glass of the prior art described in Example 3 with solid glass droplets adhered to the ampoule.
  • FIG. 12 is an infrared photograph of a human hand and fingers viewed through an As 39 S 61 glass of the prior art described in Example 3 showing striae within the glass.
  • FIG. 13 is a schematic overview of the glass fining and cooling steps of the prior art described in Examples 4 and 5.
  • a sealed ampoule and the glass melt contained therein is inside a rocking furnace positioned vertically.
  • the temperature profile is measured by thermocouples positioned along the length of the ampoule.
  • FIGS. 14A and 14B are infrared photographs comparing the clarity of As 39 Se 61 glass of the present invention with that of the prior art.
  • FIG. 14A shows a human hand viewed through As 39 Se 61 glass of the present invention.
  • FIG. 14B shows a human hand viewed through As 39 Se 61 glass of the prior art.
  • the present invention provides a method for synthesizing high optical quality multicomponent chalcogenide glasses without refractive index perturbations due to striae, phase separation or crystal formation.
  • the method uses a sealed ampoule (typically a quartz or silica ampoule) with chemical components enclosed inside, a two-zone furnace, a convection heating/mixing step, and multiple fining steps. Initially, the sealed ampoule is oriented vertically within the two-zone furnace and heated to melt the chemical components contained within, and a temperature gradient (in the vertical direction) is created between the top zone and the bottom zone such that the bottom zone has a higher temperature. This temperature gradient causes convection currents within the viscous liquid until it is sufficiently mixed due to the convective flow.
  • a sealed ampoule typically a quartz or silica ampoule
  • the temperature gradient is reversed such that the top zone now has a higher temperature and the convective flow ceases.
  • the furnace temperatures are then reduced over a period of time, with holds at multiple temperatures for fining (removal of bubbles) and cooling to form a solid glass.
  • FIG. 1 shows vapor pressure (in Pascal, Pa.) curves of five elements commonly used in the production of chalcogenide glasses for infrared applications: sulfur 80 , selenium 81 , arsenic 82 , tellurium 83 , and germanium 84 as a function of temperature in degrees centigrade (° C.). The curves are plotted in logarithmic scale on both axes.
  • FIG. 2 The glass melting and homogenization step of the process to synthesize infrared glasses by melt processing using the present invention without mechanical agitation is shown schematically in FIG. 2 .
  • a sealed ampoule 10 containing a glass melt 11 is placed inside a furnace 12 having two separate heating elements, specifically a top, or upper, heating element 13 and a bottom, or lower, heating element 14 . These heating elements are controlled separately such that they provide two distinct temperature zones within the furnace, a top, or upper, zone 15 and a bottom, or lower, zone 16 .
  • the furnace and ampoule are oriented such that their common central axis 17 is approximately vertical and stationary.
  • the bottom zone is heated to be significantly hotter than the top zone, in the example presented here, the temperature difference between the two zones is about 350° C.
  • the temperature gradient imparts a convective flow 18 within the glass melt 11 that acts to mix the glass melt 11 .
  • Chemical elements leave the surface of the glass melt 19 , and enter the vapor phase 20 above the glass melt 11 .
  • Liquid droplets 21 containing the glass constituents condense on the upper portion of the ampoule 10 and may fall as condensation droplets 22 back into the glass melt 11 where they are reincorporated therein.
  • Bubbles 23 may form within the glass melt 11 or at the ampoule-glass melt interface and migrate through the glass melt 11 to the surface 19 .
  • the migration of bubbles 23 through the glass melt 11 can be vigorous and provide significant mixing and agitation to the glass melt 11 without external mechanical agitation, for example rocking, inversion or shaking of the furnace 12 , the ampoule 10 , or both.
  • This invention has been demonstrated using As 39 Se 61 glasses in the examples but can also be applied to other two-component and multi-component chalcogenide glasses such as but not limited to arsenic, sulfur, selenium and tellurium based glasses and other multi-component chalcogenide and chalcohalide glasses containing germanium, antimony, gallium aluminum, indium, bismuth, tin, iodine, bromine, chlorine, fluorine, lanthanum, and other elements.
  • the present invention could also be applied to the fabrication of other glasses (for example silicates, borates, fluorides, phosphates, and others) or processing of viscous liquids (for example polymer melts, metals, salts, and other liquids) where homogeneity is desired.
  • other glasses for example silicates, borates, fluorides, phosphates, and others
  • viscous liquids for example polymer melts, metals, salts, and other liquids
  • Example 1 Process of the Present Invention to Make Striae-Free and Crystallite-Free As x Se y Glass Without Mechanical Rocking or Stirring of the Glass Melt
  • the ampoule was connected to a vacuum pump, evacuated, sealed, and placed inside a fixed vertical cylindrical bore, or tube, furnace 12 having an upper furnace element 13 and a lower furnace element 14 respectively corresponding to a top heat zone 15 and a bottom heat zone 16 , and where each furnace zone was heated according to a glass melting schedule, an example of which is shown for As 39 Se 61 glass in Table 1.
  • the furnace in this example is fixed in a vertical position throughout the process, with no rocking or external mechanical agitation.
  • Step 1 shown schematically in FIG. 2 , the bottom zone 16 and top zone 15 of the furnace 12 were heated at a rate of +2° C./min from 20° C. (room temperature) to 800° C. and 450° C., respectively, to establish a negative temperature gradient (wherein the bottom of the furnace is hotter than the top), and these temperatures were held for 24 hours.
  • the chemical constituents, As and Se were heated to form a glass melt 11 , and the large temperature gradient (350° C.) caused significant convective currents 18 within the glass melt 11 .
  • the equilibrium vapor pressures of selenium 81 and arsenic 82 are significantly higher at 800° C., the temperature of the glass melt, than they are at 450° C., the temperature of the top of the ampoule, supporting the continuous evaporation-reflux cycle during this step.
  • Steps 2-7 are shown schematically in FIG. 3 , which is a schematic overview of the glass fining and cooling steps of the process to synthesize infrared glasses by melt processing using the present invention.
  • a sealed ampoule 10 containing a well-mixed, homogenized glass melt 24 is vertically oriented inside a furnace 12 having two separate heating elements, specifically a top, or upper, heating element 13 and a bottom, or lower, heating element 14 . These heating elements are controlled separately such that they provide two distinct temperature zones within the furnace, a top, or upper, zone 15 and a bottom, or lower, zone 16 .
  • the furnace 12 and ampoule 10 are oriented such that their common central axis 17 is approximately vertical and stationary.
  • Thermocouples 101 , 102 , 103 , 104 , 105 are affixed to the ampoule 10 at five different positions to monitor the temperature of the ampoule 10 during the process. After the mixing step, the temperature gradient is reversed such that the top zone is hotter than the bottom zone to eliminate the convective currents and stop mixing of the glass.
  • Step 2 serves to stop the homogenization process and start the fining, or bubble removal process in preparation for cooling and solidification.
  • the top zone 15 of the furnace was heated at a rate of +0.6° C./min to 750° C.
  • the bottom zone 16 was cooled at a rate of ⁇ 0.6° C./min to 650° C. to establish the positive temperature gradient of 100° C. between the top and bottom zones. Because the temperature gradient is now inverted from the prior step, the convective current slows and eventually stops when thermal equilibrium is reached. Also, the equilibrium vapor pressures of arsenic 82 and selenium 81 (see FIG.
  • Step 1 are now lower in the glass melt, 650° C., than near the top of the ampoule, 750° C.; and condensation at the top of the ampoule stops, ending the vigorous evaporation/condensation cycle.
  • Steps 3-7 serve to cool the glass melt slowly, while maintaining a positive temperature gradient (where the top is hotter than the bottom) to prevent unwanted convective currents. Slow cooling minimizes the rate of condensation at the top of the ampoule.
  • Step 3 the top zone and bottom zone were cooled at a rate of ⁇ 0.6° C./min to 650° C. and 550° C., respectively, and held for 5 hours while maintaining a temperature gradient of 100° C. between the top and bottom zones.
  • Step 4 the top zone and bottom zone were cooled at a rate of ⁇ 0.6° C./min to 550° C. and 450° C., respectively, and held for 5 hours while maintaining a temperature gradient of 100° C. between the top and bottom zones.
  • Step 5 the top zone and bottom zone were cooled at a rate of ⁇ 0.6° C./min to 450° C. and 350° C., respectively, and held for 5 hours while maintaining a temperature gradient of 100° C. between the top and bottom zones.
  • Step 6 the top zone and bottom zone were cooled at a rate of ⁇ 0.6° C./min to 350° C. and 250° C., respectively, and held for 5 hours while maintaining a temperature gradient of 100° C. between the top and bottom zones.
  • Step 7 the top zone and bottom zone were cooled at a rate of ⁇ 0.6° C./min to 250° C. and 150° C., respectively, and held for 0.5 hours while maintaining a temperature gradient of 100° C. between the top and bottom zones.
  • the temperature profile of the ampoule during the dwell portion of Step 7 was measured with thermocouples 101 , 102 , 103 , 104 , 105 ( FIG. 3 ) and is reported in Table 2.
  • FIG. 4 shows the temperature profiles of the furnace top zone 74 , the furnace bottom zone 75 , and the temperature difference between the furnace top and bottom zones, or the temperature gradient 76 , during the synthesis of the As 39 Se 61 glass of Example 1, Steps 1 through 7.
  • a negative gradient or inverse gradient is used for the first ⁇ 35 hours of the example and indicates that the temperature of the bottom zone is greater than that of the top zone.
  • a positive gradient indicates that the temperature of the top zone is greater than that of the bottom zone and reduces the drive for convective currents in the glass melt.
  • a positive gradient is used in this example from about hour 35 until the ampoule is removed for quenching in Step 8.
  • Step 8 the hot ampoule is removed from the furnace, submerged in a room temperature ( ⁇ 25° C.) water bath for between 1-2 seconds to quench the glass melt into a solid glass.
  • Step 9 the ampoule and glass contained within are placed in a second vertically oriented cylindrical bore, tube, furnace at 165° C. for 10 hours to anneal or remove stress introduced by quenching the solid glass. After 10 hours, the furnace, ampoule, and glass are then cooled to room temperature and removed from the annealing furnace.
  • FIG. 5A shows a photograph of the solid glass ingot 25 within the ampoule 10 . Inside the sealed ampoule 10 is a homogeneous, striae-free, infrared glass ingot 25 with a composition of 39% atomic As and 61% atomic Se. The surface of the ampoule above the glass ingot is clean and free of condensed beads of glass.
  • the glass ingot 25 was removed from the ampoule 10 , and cut at the marked positions 26 , 27 in FIG. 5A . As shown in FIG. 5B , the top and bottom faces of the resulting As—Se glass cylinder 28 were ground flat and parallel and subsequently polished.
  • FIG. 6A shows an infrared photograph of a heated perforated steel plate 29 viewed through the As—Se glass cylinder with polished end faces 28 .
  • FIG. 6B shows an infrared photograph of a human hand 31 viewed through the As—Se glass cylinder with polished end faces 28 , where the camera is focused on the distal surface of the glass cylinder. There are no detectable striae, inhomogeneity, or refractive index perturbations in the bulk glass.
  • the glass transition temperature of a 2-component glass such as the As 39 Se 61 glass of this example, is very sensitive to the chemical makeup of the glass such that a 1% atomic difference in composition can result in a 10° C. difference in glass transition temperature. Measurement of glass transition temperature is therefore often used as a surrogate for compositional variation in glass making.
  • the As 39 Se 61 glass from this example shown schematically in FIG. 7 , was cut into pieces and the glass transition temperature was measured at the 10 locations indicated in FIG. 7 .
  • Example 2 Process of the Present Invention to Make Striae-Free and Crystallite-Free As 39 S 56 Se 5 Glass Without Mechanical Rocking or Stirring of the Glass Melt
  • Elemental chemicals arsenic, sulfur, and selenium sufficient to make 80 g of As 39 S 56 Se 5 glass (percentages atomic basis) were placed inside an ampoule under a dry nitrogen atmosphere.
  • the ampoule was heat sealed under vacuum using an oxygen-methane flame and placed inside a two-zone vertical tube furnace having an upper heating element and a lower heating element as shown in FIG. 2 .
  • the chemicals were melted, forming a glass melt, and cooled to form a solid glass according to the schedule in Table 4.
  • Step 1 when the top zone temperature was about 550° C. and the bottom zone temperature was about 800° C., a vigorous continual evaporation-reflux cycle could be observed visually and audibly.
  • FIG. 1 when the top zone temperature was about 550° C. and the bottom zone temperature was about 800° C.
  • FIG. 8 shows graphically the temperature profiles of the furnace top zone 71 , the furnace bottom zone 72 , and the temperature difference between the furnace top and bottom zones, or the temperature gradient 73 , during Steps 1 through 4 of the process in this Example.
  • the equilibrium vapor pressures of the constituent elements, arsenic 82 , sulfur 80 , and selenium 81 are lower at the top of the ampoule than in the glass melt throughout Steps 1 and 2 driving the evaporation/condensation reflux cycle that encourages rapid mixing of the melt.
  • a negative gradient or inverse gradient is used for the first ⁇ 4.5 hours of the example and indicates that the temperature of the bottom zone is greater than that of the top zone.
  • Vigorous evaporation/reflux cycling is observed visually and audibly in this Example when the magnitude of the inverse temperature gradient exceeds about 250° C., in other words from about hour 2 though about hour 3.5.
  • a positive gradient indicates that the temperature of the top zone is greater than that of the bottom zone and reduces the drive for convective currents in the glass melt.
  • a positive gradient is used in this example from about hour 4.5 until the ampoule is removed for quenching in Step 5.
  • the resulting glass was free of striae and transmitted infrared light.
  • FIG. 9 is a schematic overview of the glass melting and homogenization step used in the processes to synthesize infrared glasses by melt processing in the prior art (Examples 3, 4, and 5).
  • the rocking furnace 48 comprises an upper heating element 49 and a lower heating element 50 .
  • the furnace elements are not necessarily independently controlled.
  • the furnace elements may be independently controlled, but are set to the same temperature during the homogenization step.
  • a sealed quartz ampoule 51 with a glass melt 52 therein is positioned within the furnace 48 such that the furnace 48 and ampoule 51 share a common central axis 53 .
  • the furnace 48 , the ampoule 51 , and the glass melt 52 are rocked through a range of motion 54 of about ⁇ 45° with a center of rotation 55 located within the center of the furnace 48 .
  • the melting/homogenization step of this Example 3 is shown schematically in FIG. 9 .
  • Arsenic and sulfur precursors required to make a glass with the composition of 39% atomic As and 61% atomic S were loaded in a silica ampoule 51 under an inert nitrogen gas atmosphere.
  • the ampoule 51 was connected to a vacuum pump, evacuated, sealed, and placed inside a rocking furnace 48 with a ⁇ 45° angle of inclination 54 where it was heated and rocked according to a glass melting schedule, an example of which is shown for As 39 S 61 glass in Table 5 (Sanghera et al., “Development of Low-Loss IR Transmitting Chalcogenide Glass Fibers,” SPIE vol.
  • the rocking furnace 48 may have one or more furnace elements that may or may not be controlled independently of one another.
  • the furnace 48 is shown to have two heating elements: and upper or top element 49 and a lower or bottom element 50 corresponding to a top and bottom heating zone respectively.
  • the sealed silica ampoule 51 and the chemicals within were inserted into the rocking furnace 48 such that they share a common central axis 53 , and the lengthwise center of the ampoule 55 corresponds to the center of rotation of the rocking furnace.
  • Step 1 as shown schematically in FIG. 9 and detailed in Table 5, the top zone and bottom zone of the furnace were controlled together, as is commonly done in the prior art, and heated at a rate of 3° C./min from 20° C. (room temperature) to 750° C. at which point the rocking action of the furnace was started. The furnace then remained at 750° C. for 10 hours and was actively rocked at an inclination angle of ⁇ 45° to facilitate mixing and homogenization of the glass melt 52 .
  • Step 2 the fining step, the furnace motion was stopped and the furnace was set to a vertical position (90° fixed angle), as shown schematically in FIG. 10 , and held at temperature (750° C.) for 1 hour to facilitate fining and settling of the glass melt. It is in this step that any bubbles introduced by rocking migrate to the surface.
  • FIG. 10 is a schematic overview of the glass fining and cooling steps of the process to synthesize infrared glasses by melt processing in the prior art.
  • the sealed quartz ampoule 51 containing a well-mixed, homogenized glass melt 56 is inside a rocking furnace 48 that is in a vertical position.
  • the rocking furnace 48 comprises one or more heating elements 49 , 50 that, in this case, are controlled only as a group and are set to the same temperature or are set to a temperature difference of less than about 50° C.
  • Convective heat loss within the furnace 57 ensures that the upper portion of the furnace 58 is cooler than the lower portion 59 and imparts a temperature gradient.
  • the glass melt at the bottom of the ampoule retains heat and cools more slowly than the portion of the ampoule above the melt, resulting in a cooling lag that contributes to the temperature gradient.
  • the temperature gradient was measured using thermocouples affixed to the ampoule 51 at five positions 201 , 202 , 203 , 204 , 205 during the process. During the fining step, this temperature gradient imparts convective currents 60 within the glass melt 56 which has a low viscosity. Chemical elements leave the upper surface 61 of the glass melt, and enter the vapor phase 62 above the glass melt.
  • the top portion of the silica ampoule cools faster than the bottom portion of the silica ampoule that is in contact with the glass melt 56 , which has a high thermal mass, and as such, liquid droplets 63 containing the glass constituents condense on the upper portion of the silica ampoule 51 .
  • These droplets have a different composition than the overall glass melt and may fall 64 back into the glass melt. This condensation occurs through the duration of the cooling step while the viscosity of the glass melt gradually increases. The droplets that fall later in the process are less likely to be completely reincorporated back into the glass melt resulting in striae and other inhomogeneity in the final glass.
  • Step 3 the cooling step, the temperatures of both the top and bottom zones were reduced at a rate of 5° C./min to 440° C. and the temperature was held at 440° C. for 2 hours while monitoring the temperature profile of the ampoule at five locations along its length 201 , 202 , 203 , 204 , 205 ) as indicated in FIG. 10 .
  • the temperature profile measurement shown in Table 6, indicates an inverse temperature gradient, where the bottom is hotter than the top, of ⁇ 12° C., in spite of the two furnace zones being set to the same temperature.
  • Step 3 Measurement point (See FIG. 10) Temperature 201 430° C. 202 435° C. 203 438° C. 204 441° C. 205 442° C.
  • Step 4 the quench step, the hot ampoule and the glass melt contained therein were removed from the furnace and submerged in a room temperature (about 25° C.) water bath for 30 seconds to rapidly cool the melt forming a solid glass.
  • Step 5 the annealing step, the ampoule and the solid glass ingot contained therein were then placed in another furnace at 180° C. for 10 hours to remove residual stress introduced by the rapid cooling in Step 4 from the solid glass.
  • FIG. 11 shows a photograph of the solid glass ingot 66 within the sealed silica quartz ampoule 65 .
  • the surface of the ampoule above the glass ingot has condensed beads of glass 67 and soot adhered to it.
  • the glass ingot was removed from the ampoule, and the top and bottom portions of the ingot were removed using a diamond saw and the end faces of the resulting As 39 S 61 glass cylinder were ground flat and polished.
  • the glass was inspected using an infrared camera.
  • FIG. 12 shows an infrared image of a human hand and fingers 68 viewed through the 65 mm thick glass cylinder 69 and reveals significant striae and optical inhomogeneity 70 .
  • Step 3 of this example although the top and bottom zones of the furnace are both set at the same temperature, 440° C., the actual measured temperature along the length of the ampoule containing the glass melt varies due to 1) convective heat loss within the furnace and 2) the cooling lag resulting from the larger thermal mass of the glass melt in the bottom of the ampoule compared to the rapidly cooling portion of the ampoule above the glass melt.
  • a temperature gradient ( ⁇ T) of 12° C. has been measured in the example as detailed in Table 6. This gradient causes thermal convection currents 60 within the glass melt 56 when the temperature is high and the glass melt viscosity is low ( FIG. 10 ).
  • the vapor pressures of arsenic and sulfur are high and both elements are present as gasses 62 in the atmosphere above the glass melt within the sealed ampoule ( FIG. 10 ).
  • the equilibrium vapor pressures for both elements decrease and droplets of liquid As—S 63 condense on the cooler upper portion of the ampoule.
  • the glass melt, as it cools, is still hotter than the top of the ampoule, material may continue to evaporate from its surface 61 continuing the evaporation/condensation cycle. These glass droplets may then drip back into the melt 56 .
  • the condensation droplets 64 may have a different composition than the rest of the glass melt, and this continual mass fluxing cycle can cause a compositional non-uniformity throughout the entire melt as thermal convection currents 60 distribute them deeper into the glass melt.
  • the composition of the glass near the surface is changing as condensation of gaseous components (e.g. sulfur) from the closed system settle on the surface of the glass melt forming a depletion layer.
  • Gaseous components e.g. sulfur
  • Thermal convection currents within the glass allow this depletion layer, with a slightly different composition, to become somewhat reincorporated into the bulk glass. The convection currents are not sufficient to thoroughly distribute or homogenize the glass, resulting in compositional gradients within the glass.
  • Step 4 During water quenching of Step 4, it took between 26-30 seconds to quench the glass. The viscosity of the glass increased rapidly as the glass melt cooled and the compositional gradients became frozen resulting in striae in the bulk glass. The viscosity of the glass during the annealing step, Step 5, is too high to allow any removal of compositional non-uniformity. Consequently, there are refractive index perturbations in the striae-containing glass that degrade the quality of the glass and resulting optics and optical fiber made from the glass of this prior art example.
  • Nguyen et al. in a prior art invention teach a method to synthesize striae-free arsenic sulfide-based chalcogenide glass (As 39 S 61 ) and other chalcogenide glasses (Nguyen et al., “Striae-Free Chalcogenide Glasses,” U.S. Pat. No. 9,708,210 (Jul. 18, 2017)).
  • Their invention comprises six steps as detailed in Table 7, and uses a two-zone rocking furnace 48 as shown schematically in FIGS. 9 and 13 , with an upper zone 58 and a lower zone 59 .
  • the key feature of this prior art method is the establishment of a controlled temperature gradient within the furnace, in order to suppress the convection currents within the glass melt and the evaporation-condensation cycle that introduces striae.
  • the temperature of the upper zone is hotter than the lower zone by 100° C. during steps 1-4 of the melting schedule shown here in Table 7 (Nguyen et al., “Striae-Free Chalcogenide Glasses,” U.S. Pat. No. 9,708,210 (Jul. 18, 2017)).
  • the temperature gradient along the length of the ampoule 51 was measured at five locations 201 , 202 , 203 , 204 , 206 and is shown in Table 8. This positive temperature gradient eliminates the main causes of striae and therefore reduces compositional variations in the molten glass but requires mechanical agitation of the glass melt, by means of an oscillatory rocking furnace in this example.
  • Step 4 Measurement point (See FIG. 13) Temperature 201 361° C. 202 360° C. 203 262° C. 204 261° C. 206 260° C.
  • Nguyen et al. in another prior art invention teach a method to synthesize striae-free and crystallite-free Ge x As y S (100-x-y-z) Se z glasses (Nguyen et al., “Manufacturing Process for Striae-Free Multicomponent Chalcogenide Glasses via Multiple Fining Steps,” U.S. patent Ser. No. 10/131,568 (Nov. 20, 2018)).
  • Their invention builds upon the prior art in Example 4, comprises nine steps as detailed in Table 9, and uses, a two-zone rocking furnace 48 as shown schematically in FIGS. 9 and 13 , with an upper zone 58 and a lower zone 59 .
  • FIG. 13 is a schematic overview of the glass fining and cooling steps of the process to synthesize infrared glasses by melt processing.
  • the sealed quartz ampoule 51 containing a well-mixed, homogenized glass melt 56 is inside a rocking furnace 48 that is in a vertical position.
  • the rocking furnace 48 comprises an upper or top heating element 49 and a lower or bottom heating element 50 that are independently controlled, and in these prior art examples are set to different temperatures during the glass fining and cooling steps such that the temperature of the upper or top zone 58 is about 100° hotter than the lower or bottom zone 59 .
  • the temperature gradient was measured using thermocouples affixed to the ampoule 51 at five positions 201 , 202 , 203 , 204 , 206 during the process. During the fining step, this temperature gradient prevents convective currents within the glass melt 56 .
  • steps 3-7 the temperature of the upper zone is hotter than the lower zone by 100° C.
  • This method also provides multiple cooling steps, steps 5-7, with rapid cooling rates between them. Rapid cooling between these steps prevents formation of crystallites by cooling quickly through the temperatures where crystals nucleate.
  • the temperature profile of the cooling is shown in Table 10.
  • Step 7 Measurement point (See FIG. 13) Temperature 201 451° C. 202 450° C. 203 352° C. 204 351° C. 205 350° C.
  • FIGS. 14A and 14B are infrared photographs comparing the clarity of As 39 Se 61 glass of the present invention with that of the prior art.
  • FIG. 14A shows a human hand 31 viewed through a 55 mm thick As—Se glass cylinder with polished end faces 28 of the present invention where the camera is focused on the distal surface of the glass cylinder showing no striae or optical inhomogeneity within the glass.
  • FIG. 14B shows a human hand 31 viewed through a 55 mm thick As—S glass cylinder with polished end faces 46 of the prior art as detailed in Example 3 where the camera is focused on the distal surface of the glass cylinder revealing significant striae or optical inhomogeneity 47 .

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US10131568B2 (en) * 2015-03-03 2018-11-20 The United States Of America, As Represented By The Secretary Of The Navy Manufacturing process for striae-free multicomponent chalcogenide glasses via multiple fining steps
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